1. Technical Field
The present invention relates to a MEMS vibrator and an oscillator.
2. Related Art
MEMS (Micro Electro Mechanical Systems), which are one of techniques for forming a minute structure, mean a technique for making a micro electro mechanical system in the order of microns, or the product thereof, for example.
JP-A-2010-162629 discloses a MEMS vibrator having a fixed electrode and a movable electrode, in which the movable electrode is driven by electrostatic force generated between the electrodes. Output of such an electrostatic-type MEMS vibrator is generated by a change in capacitance between the fixed electrode and the movable electrode when the MEMS vibrator is driven. Therefore, the larger the crossing area (area of a region where the electrodes overlap each other) of the fixed electrode and the movable electrode is, the higher the output is.
On the other hand, a driving frequency is a natural frequency of a vibrator, and determined by the shape and dimensions of the vibrator. Various shapes of MEMS vibrators driven in a frequency band from several kHz to several GHz have been proposed so far. In the case of the vibrator having a cantilevered structure disclosed in JP-A-2010-162629, the driving frequency is determined by the length and thickness of a beam portion (movable electrode). When the thickness is constant, the great length of the beam portion makes the frequency low, and the small length thereof makes the frequency high.
However, when it is intended to drive the vibrator at a high frequency, the crossing area of the fixed electrode and the beam portion is reduced because of the small length of the beam portion, resulting in low output in some cases. This means that when the vibrator is considered as an equivalent circuit, the series resistance is high. When the MEMS vibrator is used for an oscillator for example, phase reversal is insufficient as the resistance is increased, failing to satisfy oscillation conditions in some cases.